1. Field of the Invention
The present teachings relate to a technique for welding workpieces by employing electric resistance heating caused by conducting electric current through the workpieces.
2. Description of the Related Art
Known welding systems of this type generally comprise a pair of electrodes for retaining workpieces therebetween, an electric power source for generating a welding current that can be conducted through the workpieces by the pair of electrodes, and a controller for controlling the electric power source and the current generated thereby. In order to produce a sturdy weld, the welding power must be controlled. If the welding power applied to the workpieces is too great, the weld site may spatter and thereby prohibit a sturdy weld. On the other hand, if the welding power is too small, insufficient heating at the weld site reduces the weld quality. Previously, welding power has been controlled by adjusting either welding current or welding voltage.
Various kinds of predetermined values have been utilized to control the amount of power that is applied to the weld site. For example, welding current generally is used as suchapredetermined value. In this case, the welding current is monitored and increased if it is too little to produce a sturdy weld. The welding current is reduced if the welding power is too great to prevent overheating of the workpiece.
However, various kinds of workpieces must be welded and an optimum value or a standard predetermined value depends upon the composition of the workpiece materials, including whether the workpieces are coated or not. In known welding techniques, when the welding current is used as the predetermined value, predetermined welding current values may be too great or too little as a result of variation in workpiece compositions. Therefore, a standard predetermined current value must be determined independently for every kind of workpieces that will be welded. In application of these known welding techniques, the determination of the standard current value typically requires a large number of experiments.
Further, even when the standard value of the predetermined value has been predetermined based upon experimental results, compensation for the standard value must be provided as welding operations progress, because the resistance of the welding electrodes usually changes after repeated welding operations. Once the electrodes have been worn, a sturdy weld can not be made simply by controlling the welding current based on the standard value that was determined before the electrodes were worn by repeated welding operations. Thus, the determination of a technique for compensating for the standard value in compliance with wear of the electrodes also has been known to require a large number of experiments.